State-of-the-art lithium-ion cells have a lithiated carbon negative electrode, or anode, (LixC6) and a lithium-cobalt-oxide positive electrode, or cathode, Li1−xCoO2. During charge and discharge of the cells lithium ions are transported between the two host structures of the anode and cathode with the simultaneous oxidation or reduction of the host electrodes, respectively. When graphite is used as the anode, the voltage of the cell is approximately 4 V. The cathode material LiCoO2, which has a layered structure, is expensive and becomes unstable at low lithium content, i.e., when cells reach an overcharged state at x≧0.5. Alternative less expensive electrode materials that are isostructural with LiCoO2, such as LiNi0.8Co0.2O2 and LiNi0.5Mn0.5O2 are being developed in the hope of replacing at least part of the cobalt component of the electrode. However, all these layered structures, when extensively delithiated are unstable, because of the high oxygen activity at the surface of the particles; therefore, the electrode particles tend to react with the organic solvents of the electrolyte or lose oxygen.
Spinel electrodes, such as those in the manganese-based system Li1+xMn2−xO4, are particularly attractive alternatives to LiCoO2 because, not only are they relatively inexpensive, but they are thermally more stable than Li1−xCoO2 or Li1−xNi0.8Co0.2O2 at low lithium loadings, and because they do not contribute to the impedance rise of electrochemically cycled lithium-ion cells to the same extent as Li1−xCoO2 or Li1−xNi0.8Co0.2O2 electrodes.
The Li1−x[Mn2]O4 spinel system has been investigated extensively in the past as an electrode for lithium-ion batteries. A major reason why the spinel system has not yet been fully commercialized is because the electrode is unstable in the cell environment, particularly if the operating temperature of the cells is raised above room temperature, for example, to 40–60° C. It is now generally acknowledged that the solubility of Lix[Mn2]O4 electrodes in acid medium occurs by the disproportionation reactionMn3+(solid)→Mn4+(solid)+Mn2+(solution)  (1)during which the Mn2+ ions go into solution, and the Mn4+ ions remain in the solid spinel phase. Such a reaction can occur in lithium-ion cells because the hydrolysis of fluorinated lithium salts such as LiPF6 with small amounts of residual water in the organic-based electrolyte solvents can generate hydrofluoric acid, HF.
Full electrochemical delithiation of Li[Mn2]O4 leaves λ-MnO2 with the [Mn2]O4 spinel framework. Like many manganese dioxides, λ-MnO2 is a powerful oxidizing agent and can be readily reduced. Therefore, any oxygen that may be evolved at the particle surface of the spinel electrode at the top of charge will result in Mn3+ ions at the electrode surface; the instability of Mn3+ ions at the high potential of the charged cell will also drive the disproportionation reaction (1) shown above, thus damaging the spinel surface and resulting in some irreversible capacity loss of the cell.
The presence of tetragonal Li2[Mn2]O4 has also been observed in very small amounts at the surface of Li[Mn2]O4 spinel electrodes at the end of discharge after high rate cycling (C/3 rate) between 4.2 and 3.3 V vs. Li. The compound Li2[Mn2]O4 in which all the manganese ions are trivalent will be unstable, like Li[Mn2]O4, at high potentials in a 1 M LiPF6/EC/DMC electrolyte that contains HF, particularly if the lithium cells are operated at 40–50° C. In this case, a disproportionation reaction occurs in which MnO dissolves from the particle surface to leave an insoluble and stable Li2MnO3 rock-salt phase. This reaction may account for some of the capacity loss of 4-V Li/Lix[Mn2]O4 cells on long-term cycling.
Substantial efforts have already been made in the past to overcome the solubility problems associated with the Li[Mn2]O4 spinel electrode. For example, partial substitution of the manganese ions in Li[Mn2]O4 with a mono-, di- or trivalent ion changes the composition of the electrode and increases the average oxidation state of the manganese ions above 3.5, thus reducing the amount of Mn3+ ions in the fully discharged electrode. Other approaches to suppress manganese dissolution from the spinel electrode have been taken, for example, by protecting the spinel particles with a surface coating, such as a low-melting lithium borate glass or a coating of LiCoO2 applied at high temperature (e.g., 700–800° C.) both of which are known to be more resistant to dissolution in the electrolytes than Li[Mn2]O4. Alternatively, a coating of ZrO2 or Co3O4 has been applied to the electrode particles. Although some success has been achieved by using these approaches, the problems of electrode instability have not yet been fully resolved and further improvements are necessary.
LiMn2O4 spinel electrodes have a tendency to lose oxygen or react with the electrolyte if charged to a high potential, such as 4.5 V, which causes irreversible capacity loss effects. Moreover, the loss of oxygen from the electrode can also contribute to exothermic reactions with the electrolyte and with the lithiated carbon negative electrode, and subsequently to thermal runaway if the temperature of the cell reaches a critical value. There is therefore a strong requirement to improve the state-of-the-art protective coatings on these electrodes to improve the overall performance and safety of lithium-ion cells.